This invention relates to spread-spectrum communications and more particularly to a system and method for locating within a cell, a remote unit communicating synchronously with a spread-spectrum-communications signal using a reference carrier signal supplied on a spread-spectrum channel by the transmitter.
Referring to FIG. 1, message data, d(t), are processed by spread-spectrum modulator 51, using a message-chip-code signal, g1(t), to generate a spread-spectrum data signal. The spread-spectrum data signal is processed by transmitter 52 using a carrier signal at a carrier frequency fo, and transmitted over communications channel 53.
At a receiver, a spread-spectrum demodulator 54 despreads the received spread-spectrum signal, and the message data are recovered by synchronous data demodulator 60 as received data. The synchronous data demodulator 60 uses a reference signal for synchronously demodulating the despread spread-spectrum signal. The square-law device 55, bandpass filter 56 and frequency divider 57 are well known in the art for generating a reference signal from a received modulated data signal. A Costas Loop or other reference signal generating circuit is adequate for this purpose.
In a fading channel, such as the ionosphere or any channel containing multipath, or more generally, any channel in which the received signal""s amplitude fluctuates with time, synchronous demodulation is not practical since the phase of the incoming signal typically is not the same as the phase of the reference. With DPSK the received signal is delayed by one symbol and multiplied by the undelayed signal. If the resulting phase is less than xc2x190xc2x0 a 0-bit is declared, otherwise a 1-bit is declared. Such a system is complex and suffers degradation of about 6 dB at error rates of 10xe2x88x922.
The prior art does not provide a system and method for synchronously communicating, using spread-spectrum modulation, with a base station and in combination locating a remote unit within the cell of a base station.
A general object of the invention is a geolocation system and method that can be used as a personal communications service.
An object of the invention is a system and method for synchronously communicating a modulated-data signal embedded in a CDMA signal, and for geolocating a remote unit, which performs well whether or not the signal is fading.
Another object of the invention is a geolocation system and method which uses a separate spread-spectrum channel as a pilot signal for a data link for geolocating a remote unit and for demodulating a modulated-data signal embedded in a CDMA signal.
An additional object of the invention is synchronous spread-spectrum-communications and geolocation system.
According to the present invention, as embodied and broadly described herein, a spread spectrum code division multiple access (CDMA) communications and geolocation system and method for use over a communications channel is provided comprising at least one base station and a plurality of remote units. The remote units may be mobile or in a fixed, stationary location. Message data are communicated between the base stations and the remote units. Message data includes, but are not limited to, digitized voice, computer data, facsimile data, video data, etc. The base station communicates base-message data to the plurality of remote units. A remote unit communicates remote-message data to the base station. Base-message data are defined herein to be message data originating from a base station, and remote-message data are defined herein to be message data originating from a remote unit.
Each of the base stations includes base-spreading means, base-generic means, base-combiner means, base-transmitter means, and base antenna. The term xe2x80x9cbasexe2x80x9d is used as a prefix to indicate an element is located at the base station, or that a Signal originates from a base station. The base-spreading means spread-spectrum processes the base-message data. The base-generic means generates a base-generic-chip-code signal. The term xe2x80x9cgenericxe2x80x9d is used as a prefix to indicate that the generic-chip-code signal is an unmodulated, or low data rate, direct-sequence spread-spectrum signal, which can serve as a pilot channel. The pilot channel allows a user to acquire timing, and provides a phase reference for coherent demodulation. The base-combiner means combines the base-generic-chip-code signal with CDMA signal. The base-transmitter means transmits the base-CDMA signal from the base station to a remote unit. The base antenna is coupled to the base-transmitter means.
Each of the remote units includes a remote antenna, remote-detection means, remote-spreading means, remote-combiner means, and remote-transmitter means. Each remote unit also may include remote-generic means. The term xe2x80x9cremotexe2x80x9d is used as a prefix to indicate an element is located at a remote unit, or that a signal originates from the remote unit. The remote-detection means is coupled to the remote antenna. The remote-detection means detects the base-generic-chip-code signal embedded in the base-CDMA signal. Using the detected-generic-chip-code signal, the remote-detection means recovers the base-message data communicated from the base station. A remote unit can retransmit the detected-base-generic-chip-code signal, or optionally, can have remote-generic means generate a different remote-generic-chip-code signal. The remote-spreading means spread-spectrum processes remote-message data. The remote-generic-chip-code signal and the spread-spectrum-processed-remote-message data are combined by the remote-combiner means, as a remote-CDMA signal. The remote unit also includes the remote-transmitter means for transmitting the remote-CDMA signal from the remote unit to the base station.
Each of the base stations further includes base-detection means and range means. The base-detection means is coupled to the base antenna. The base detection means detects the remote-generic-chip-code signal embedded in the remote-CDMA signal. The base-detection means recovers, using the detected-remote-generic-chip-code signal, the remote message data communicated from the remote unit. Using the detected-remote-generic-chip-code signal and the base-generic-chip-code signal, the range means determines a range delay between the remote unit and the base station.
The present invention may include further the steps of spread-spectrum processing the base-message data; generating a base-generic-chip-code signal; combining the base-generic-chip-code signal with the spread-spectrum-processed-base-message data, thereby generating a base-CDMA signal; transmitting the base-CDMA signal from the base station to the remote unit; detecting the base-generic-chip-code signal embedded in the base-CDMA signal; recovering, using the detected-base-generic-chip-code signal, the base-message data; spread-spectrum processing remote-message data; generating, using the detected-generic-chip-code signal and the spread-spectrum-processed-remote data, a remote-CDMA signal; transmitting the remote-CDMA signal from the remote unit to the base station; detecting the remote-generic-chip-code signal embedded in the remote-CDMA signal; recovering, using the detected-remote-generic-chip-code signal, the remote-message data; and determining, using the detected-remote-generic-chip-code signal and the base-generic-chip-code signal, a range delay between the remote unit and the base station.
Additional objects and advantages of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.